The present invention relates to flow responsive switches and is directed more particularly to a flow responsive switch for indicating the beginning and ending of the breath of a test subject.
In evaluating the physical condition of a test subject, useful information can be derived by measuring the volume and composition of the test subject's exhaled breath. Instruments for performing these measurements usually include gas analyzers for measuring at least the concentration of oxygen and carbon dioxide in the exhaled breath. Measurements of this type may be performed under conditions which range from a condition of rest to a condition of vigorous exercise. In order to properly interpret the measurements, however, it is necessary to take into account the volume of the breath. This volume, in turn, is usually measured indirectly by measuring the rate of flow and duration of each breath.
The rate of flow of a breath is conveniently measured by directing the breath through a gas turbine which is arranged to generate a series of pulses having a repetition rate that varies with the rate of gas flow therethrough. Because of the non-linearity of gas turbines, the volume of a breath can be determined from the number of pulses it produces only by taking into account the duration of a breath. The number of pulses and the duration of that breath may, for example, be combined to produce a pulses-per-second value which can then be combined with the known characteristics of the turbine to yield the volume of that breath.
In making the above measurement it has been found that much of the error that occurs results not from the operation of the turbine, but rather from the inaccurate determinaton of the duration of the breath. An accurate determination of the end of a breath has been found to be particularly difficult because of the low rates of flow that are associated therewith when the test subject is at rest.
Prior to the present invention, it was the practice to direct a flow of bias gas through the turbine and thereby cause the latter to generate a predetermined minimum number of turbine pulses per unit time. With the use of this approach the beginning of breath is taken as the time when the pulse "frequency" increases above its minimum value, and the ending of the breath is taken as the time when the pulse "frequency" returns to that minimum value. Because the beginning of a breath is usually characterized by a relatively sudden increase in the rate of gas flow, the time at which a breath begins can be determined relatively accurately in this way. Because, however, the ending of a breath is characterized by a low and slowly changing rate of flow, the time at which a breath ends cannot be determined accurately in this way. As a result, the measured duration of a breath can be in error by several percent, causing a corresponding error in the measured volume of the breath.